arm64: defconfig: enable 48-bit virtual addresses

Some armv8 SoCs (e.g. ls2080a) have physical memory maps with discontiguous
DDR regions that require 48-bit VA to have the linear map cover the entire
range of DDR.

Signed-off-by: Stuart Yoder <stuart.yoder@nxp.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>